An important aim of the development of vacuum cleaner filter bags is to increase the life-time of the filter bags. The operating period for which a vacuum cleaner filter bag can be used until it has been filled with dust and/or the until the pores of the filter material become clogged should be as long as possible and, simultaneously, the highest possible suction power of the vacuum cleaner is aimed at for this operating period. In order to solve the resultant problems, a great variety of approaches is known from the prior art.
EP 0 960 645, for example, discloses a multi-layer structure of a vacuum cleaner filter bag in the case of which a coarse filter layer is arranged ahead of a fine filter layer in the direction of air flow. Particles sucked into the vacuum cleaner filter bag will thus first impinge on the coarse filter layer which serves to retain in particular larger particles. For this purpose, the coarse filter layer is to be provided with a comparatively high thickness and a large pore volume. Clogging up of the bag can be delayed in this way for a prolonged period of time.
EP 1 123 724 discloses a multi-layer vacuum cleaner filter bag comprising a paper support layer covered on the crude gas side by a pre-filter made of microfibres; said pre-filter is effective as a separating layer and it is adapted to be periodically lifted off the support layer. This separating layer covers the support layer completely so as to guarantee that the filter bag will be sufficiently stable and effective even if the paper layer should be fully destroyed.
U.S. Pat. No. 5,647,881 discloses a vacuum cleaner filter bag having on the inner side thereof a loose diffusion layer that covers the whole inner side of the bag. The diffusion layer serves to protect the downstream filter layer from being damaged by particles impinging thereon with high speed.
A similar aim is to be achieved in WO 93/21812. In the case of this vacuum cleaner filter bag a protective layer in the form of a strip is provided in the interior of the bag; also this protective layer is provided for preventing the vacuum cleaner filter bag from being damaged. Said protective layer is arranged such that particles entering the filter bag through an inlet opening will impinge directly onto said strip so that the subsequent layer will remain protected.